Solid urea is one of main products from a urea plant that synthesizes urea. A production step of solid urea is, usually, a process that is set on a downstream side of a urea synthesis step of a urea plant. FIG. 3 is a diagram illustrating a production step of solid urea. In FIG. 3, an aqueous urea solution sent from a urea synthesis step through a line 1 is first supplied to a concentration step A, in which moisture in the aqueous urea solution is removed to give a concentrated urea solution. Then, the concentrated urea solution is supplied to a granulation step B for producing solid urea via a line 2.
In the granulation step B, granular solid urea is produced by use of a known granulation apparatus (line 3). As a known granulation apparatus, a urea granulation apparatus using a fluidized bed or a fluidized/spouted bed or the like is employed. In place of a known granulation apparatus, a prilling urea producing apparatus (such as Prilling Tower) can be used. In the granulation step B, a concentrated urea solution introduced into the granulation apparatus is solidified/cooled by the air supplied from a line 6 to become solid urea (product solid urea). The air supplied in the granulation step B is discharged as exhaust gas (line 7). The exhaust gas in the line 7 contains urea dust generated while the concentrated urea solution was solidified/cooled. A urea concentration in the exhaust gas in the line 7 differs depending on facilities or operating conditions. The urea concentration reaches 70 to 200 mg/Nm3 in a granulation tower system, and 3000 to 10000 mg/Nm3 in a fluidized bed or fluidized/spouted bed system. Therefore, from the viewpoint of prevention of environmental pollution and economical efficiency, there is provided a urea recovery step C of recovering urea dust in the exhaust gas.
With respect to a recovery technology of urea dust in the urea recovery step C, there are, for example as a general method, a packed bed system in which fillings are packed, a venturi system. In these recovery systems, an aqueous urea solution is circulated in a washing tower, and an exhaust gas is made to contact with the aqueous urea solution. As the result of the contact with the aqueous urea solution, the urea dust in the exhaust gas is recovered in the aqueous urea solution.
The exhaust gas from which the urea dust has been recovered in the urea recovery step C is discharged into the air (line 8). The urea concentration in the exhaust gas discharged into the air has been reduced to 20 to 50 mg/Nm3. The aqueous urea solution that has recovered the urea dust in the exhaust gas in the urea recovery step C is sent to the line 1 as a recovered aqueous urea solution (line 10). Meanwhile, make-up water has been added in order to produce the recovered aqueous urea solution (line 9).
Urea in the recovered aqueous urea solution in the line 10 should be treated again in the granulation step B and be solidified/cooled. However, the recovered aqueous urea solution in the line 10 contains much moisture and therefore cannot be treated as it is in the granulation step B. Therefore, it is returned to an upstream side of the concentration step A, concentrated with moisture removed in the concentration step A, and then supplied to the granulation step B. Above is a cycle of respective steps until solid urea is produced from an aqueous solution of synthesized urea.
Incidentally, when solid urea is produced from an aqueous urea solution, it is general to add an additive to a urea solution to be supplied to the granulation step in order to enhance product quality of solid urea. For example, it is known that addition of formaldehyde as an additive leads to generation of a condensation product with urea, and that solid urea having mechanical strength can be produced. As a conventional technology, there are disclosed a method of adding an additive (a condensation product of formalin or formaldehyde with urea) to a urea solution just before the granulation step in a line 5, and concrete conditions in the granulation step when the additive is added (PTL 1). Further, it is sufficient that an additive is contained in the urea solution in the granulation step, and therefore the additive may be added before the concentration step as a line 5′.